Landracing Forum
Tech Information => Aerodynamics => Topic started by: sofadriver on January 14, 2015, 09:51:18 PM
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This has been touched on in several threads but I still have a question.
It's been said that the correct body taper to keep airflow laminar is 7 degrees. I'm sure that's correct - but at what speed? I'd think that air would always rush in to fill the void behind a passing object at the same, constant speed (regardless of the object's speed). If that's correct, then wouldn't more taper be acceptable at slower speeds where the air has more time to remain laminar?
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Aero drag can be looked at in terms of energy. Air has mass and it takes energy to accelerate it. The acceleration is a change in its velocity or in its direction of movement. The goal is to disturb the smallest mass of air possible and to minimize changing its velocity or direction of travel.
The tail works well if flow stays attached to it for its entire length. A seven degree angle has been commonly used. This minimizes the energy loss due to turbulence behind the bike. None of the references I have seen have referred to velocity vs attachment.
Another purpose of the tail is to reduce the size of the low pressure area behind the bike. It does this regardless of whether or not flow is attached near the end. There is a lot of turbulence associated with the truncated end of the teardrop shape. A recent photo posted on the new Charlie Toy thread shows a bike racing across the salt. It is a crimson or red bike, as I remember. Its tail uses a variable taper rate on both the sides and top to reduce the truncated end size. This is the best example I have seen.
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The link on reply 3 on the Charlie Toy thread shows Bike #68. A good tail on that one. Further down in the linked attachment is an electric bike. Another nice tail.
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None of the references I have seen have referred to velocity vs attachment.
Exactly, but doesn't it make sense that velocity would affect attachment?
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i think you would need to be extremely fast to start seeing your speed effect attachment
If you google and start reading on commonly used naca shapes you can find wind tunnel data and detachment points in relation to Reynolds number
Human powered LSR bicycle guys have a lot of info on shapes around naca 0024 , and wind turbine blades are often at naca 0018 (this has a bit of info available )
on a APS bike your probably limited to a naca 0021 to a 0024 unless you dont mind a larger rear surface where you cut it off
In my experiance it seems difficult to get the air to reattach after the leg cutouts , it can take up to3 feet to stabilise the flow , which just gives you a broad area for the turbulance to act on if you get it wrong
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I really feel for you guys with bikes.
Aero is no joke and lets be honest, there isn't much wind tunnel data floating around.
Sofadriver R&D, R&D and more. :cheers:
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Sofadriver said: "Exactly, but doesn't it make sense that velocity would affect attachment?" My response is to get a good book on aero, read the definition of Reynolds number until you understand it and then re-think what you said.
Rex
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mixing up my viscocity with velocitys ;)
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mixing up my viscocity with velocitys ;)
My velocity tends to suffer from viscosity.
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I'm confused on this . 66-021 profile contracts at about 16 deg per side but is one of the lowest drag profiles . How can that be ?
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on a APS bike your probably limited to a naca 0021 to a 0024 unless you dont mind a larger rear surface where you cut it off
Thanks for that tip, maj.
naca 0021 tapers at about 13% with the radius measuring 21% at 30% chord length. That's about a perfect fit.
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I'm confused on this . 66-021 profile contracts at about 16 deg per side but is one of the lowest drag profiles . How can that be ?
And that's much more than the naca 0021.
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Between .7 and .8 cord 66-021 tapers 19.563 deg .
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John,
The 66021 is one of the NACA NLF (natural laminar flow) designs. There is a great design book by a guy named Bruce Carmichael ( Personal Aircraft Drag Reduction) that discusses these kinds of things. Worth buying.
Rex
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Don't forget about the profile of the top of the bike.
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Think I figured out why 3D bodies need more gradual taper than 2D wings . The Reynolds number of a LSR body is far higher than what wing drag data shows . On the ground gives the air fewer directions than in free air to fill the hole you made .
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Veering slightly off the original topic, in my experience with sub-100mph designs (and faster is only more tricky), laminar flow is exceptionally hard to maintain on a ground vehicle particularly towards the rear. Even with the most polished, smooth, shiny body, you still have to consider that any disturbance from the forebody is going to transition the flow, be it from a bolt or a non-flush join or any angle that produces any kind of vortex. Then factor in vibrations, dirt/dust/salt/sand, etc. and I'd say realistically you're only going to stay laminar for a short run at the front and you are unlikely to get it back later.
However, I think that's not what this conversation was about - a lot of confusion can arise about what people mean when they say laminar though, as a lot of designers just mean "smooth", which often then translates to attached (and I believe this thread is about flow staying attached rather than laminar). You can have separated laminar flow, same as you can have smooth attached turbulent flow. Laminar flow next to the body is worth chasing if the rest of a design is great, as any well-designed streamlined bodywork would be looking at maybe 70% more of the drag just coming from skin friction, but like I said, in practice it can be very hard to achieve, and it is almost never determined accurately in real-world tests even if a very clean, low turbulence wind tunnel said it would feature heavily. Working to keep flow attached as long as possible is way more bang for your buck, and if you want to try a tighter angle, then you definitely want the flow to be turbulent next to the surface as it'll stay attached for longer.
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and if you want to try a tighter angle, then you definitely want the flow to be turbulent next to the surface as it'll stay attached for longer.
Thank you, Graham. Very informative.
So what's the best way to accomplish that? I've seen videos of little vortex generators that seem to work very well.
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Did a lot of research on this. It seems that the more bulbous 66-021 shape is much better in cross winds but the 0021 is fastest in line.
The real issue with any of these body shapes is whether I can copy it onto the bike while using my body as part of the shape. Guess I'll just get as close to 0021 as I can. :dhorse:
(n66021-il) NACA 66-021 AIRFOIL
NACA 66(4)-021 airfoil Max thickness 21% at 45% chord
(naca0021-il) NACA 0021
NACA 0021 airfoil Max thickness 21% at 30% chord
(joukowsk0021-jf) Joukovsky f=0% t=21%
Joukowski 21% symmetrical airfoil Max thickness 21% at 26.6% chord
(naca16021-il) NACA 16-021
NACA 16-021 airfoil Max thickness 21% at 50% chord
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.....The real issue with any of these body shapes is whether I can copy it onto the bike..
Exactly the problem with a bike or car. They aren't as simple as a section of some 'perfect' foil. They have tops and bottoms to deal with and the ground effect on the bottom along with canopies and wheels/tires and exhaust and inlets and other miscellaneous bumps and such. Then to get the thickness for the shape the overall length is likely to be too long or too short to be practical.
Probably the most successful at getting close to an ideal shape have been...
(http://images.motorcycle-usa.com/PhotoGallerys/med/Buddfab2.jpg)
http://www.motorcycle-usa.com/287/1518/Motorcycle-Article/Salt-Addiction-Buddfab-Streamliner.aspx
...Eric and John's Bubfab streamliner above (over 130 mph with 50cc) ... or ....
(http://images.motorcycle-usa.com/PhotoGallerys/xlarge/IMG_9394.jpg)
... Denis Manning's BUB Streamliner #7 ..... or .....
(http://images.motorcycle-usa.com/PhotoGallerys/Sam-EZ-Hook-DL.jpg)
...Sam's and a few others. Notice that I didn't show any cars 8-).
I think I'll stick with the simpler 7 deg. figure and say I did the best I could :-D
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Did a lot of research on this. It seems that the more bulbous 66-021 shape is much better in cross winds but the 0021 is fastest in line.
The real issue with any of these body shapes is whether I can copy it onto the bike while using my body as part of the shape. Guess I'll just get as close to 0021 as I can. :dhorse:
(n66021-il) NACA 66-021 AIRFOIL
NACA 66(4)-021 airfoil Max thickness 21% at 45% chord
(naca0021-il) NACA 0021
NACA 0021 airfoil Max thickness 21% at 30% chord
(joukowsk0021-jf) Joukovsky f=0% t=21%
Joukowski 21% symmetrical airfoil Max thickness 21% at 26.6% chord
(naca16021-il) NACA 16-021
NACA 16-021 airfoil Max thickness 21% at 50% chord
"Guess I'll just get as close to 0021 as I can. :dhorse:"
I see a streamliner in your future. :-D :cheers:
Just say it. :wink:
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it is good to have the air flow parallel to the bike centerline where it breaks off of the front edge of the rider cutout. There is not much distance to accomplish this between the tip of the nose and the back edge of the windshield and the front edge of the arm and hand cutout. The bulbous nose shape works best for this. The Triumph has the bulbous nose shape on the top where the handlebars are and a slimmer NACA shape down lower where the engine is. Another reason for the more bulbous shape is stability in side winds, as stated. It works well for this is my experience.
The air flow is not always attached behind the rider. The tail occupies a turbulent area where it provides a degree of flow organization and it lessens the power robbing big vortices. This helps to reduce energy loss.
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/quote
I see a streamliner in your future. :-D :cheers:
Just say it. :wink:
[/quote]
My priorities should I win a HUGE lottery;
1. get my name changed
2. set up trust funds
3. build at least one Ronald McDonald house (bearing my name of course)
4. visit Porsche dealer for new 911 (daily driver)
5. visit Lotus dealer for new Elise (weekend fun car)
6. make plans to build streamliner
7. find and hire gorgeous young, well endowed women with engineering degrees and excellent fab/ welding skills to build said streamliner.
got it all planned out! :cheers:
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The amount of aero streamlining that can be applied to a conventional motorcycle is limited. All the successful streamliners have a considerably longer wheel base than a sit on bike.
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Hey Mike I will supervise the girls for a can of pepsi a day. :cheers: :cheers: :cheers: :cheers:
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No way! You'll have to pay more than that! :-D
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and if you want to try a tighter angle, then you definitely want the flow to be turbulent next to the surface as it'll stay attached for longer.
Thank you, Graham. Very informative.
So what's the best way to accomplish that? I've seen videos of little vortex generators that seem to work very well.
Yes if you're just a liiiiiittle bit over an angle that might cause separation, then even a simple "trip strip" will do the job if placed a little ahead of the trouble zone, I've used everything from professional grit strips to... just a thick strip of tape. The short fin or ramp style vortex generators work well but preferably need to be designed and optimized. Really, anything physical that disturbs the flow will do the job, but that's assuming you discovered a problem you want to fix. The best thing you can do, of course, is avoid separation in the first place, which is where the careful choice of airfoil or airfoil-esque shape is most effective. In almost any instance, if you have any separation at all, it will outweigh the benefits of trying to have the least physical surface area from a drag perspective.
Thin shapes will be better in a straight line because they either present less frontal area, or disturb the flow sideways less for an equivalent-thickness bulbous shape - but the thicker shapes keep the air moving round a gentle curve at a faster rate per ft of length, which keeps the air energized and happily attached (until you push it too far, of course!) and will often perform better when it's windy. For Bonneville, you'd assume you won't run if it's more than a little windy - particularly for bikes, minimal frontal area + smooth continuous shapes (fewer junctions and angles) would be the first priority for streamlining once stability is assured.
Graham Doig
Fluids Laboratory for Interdisciplinary Projects - www.thinkflip.net
Aerospace Engineering Department
California Polytechnic State University
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Again, thank you Graham.
Well, then it seems that the area where I'm most concerned about separation (the 7" from my feet to the end of my Kamm tail) will be less worrisome due to the turbulence created by the body's foot openings. Am I getting that right? I'm trying to reduce the Kamm tail's area by tapering the last 7" more quickly.
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This magic 7 degree angle is bunk. If transition to 7 degree angle is too sharp. You will have separated flow and that is very bad. If you look at Parson Airfoils which are some of the lowest drag, the angle in the pressure recovery area is far greater than 7 degrees. If air almost stays attached or is right on the edge of staying attached at the rear of the bike, it will make for a seriously butt puckered ride as it buffers back and forth between attached and separated, right and left sides of the bike.
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Veering slightly off the original topic, in my experience with sub-100mph designs (and faster is only more tricky), laminar flow is exceptionally hard to maintain on a ground vehicle particularly towards the rear. Even with the most polished, smooth, shiny body, you still have to consider that any disturbance from the forebody is going to transition the flow, be it from a bolt or a non-flush join or any angle that produces any kind of vortex. Then factor in vibrations, dirt/dust/salt/sand, etc. and I'd say realistically you're only going to stay laminar for a short run at the front and you are unlikely to get it back later.
However, I think that's not what this conversation was about - a lot of confusion can arise about what people mean when they say laminar though, as a lot of designers just mean "smooth", which often then translates to attached (and I believe this thread is about flow staying attached rather than laminar). You can have separated laminar flow, same as you can have smooth attached turbulent flow. Laminar flow next to the body is worth chasing if the rest of a design is great, as any well-designed streamlined bodywork would be looking at maybe 70% more of the drag just coming from skin friction, but like I said, in practice it can be very hard to achieve, and it is almost never determined accurately in real-world tests even if a very clean, low turbulence wind tunnel said it would feature heavily. Working to keep flow attached as long as possible is way more bang for your buck, and if you want to try a tighter angle, then you definitely want the flow to be turbulent next to the surface as it'll stay attached for longer.
Adding to Graham's statement above, the term laminar can describe the boundary layer, a thin layer of air which transitions the air velocity from the free stream to a zero velocity at the body surface. The boundary layer may be laminar under low speed conditions for a certain distance along the body surface and then transitions to a turbulent boundary layer. The turbulent boundary layer has a more uniform average velocity and more energy and can better avoid separating from the body surface. For example, because separation is delayed, cylindrical shapes can have higher drag coefficients at low speeds than at high speeds.
For production autos a laminar boundary layer, based on Reynolds number, could exist up to about one foot along the hood of a vehicle. I used the word could because laminar boundary layers are easily tripped to be turbulent or separated. As long as the flow is attached, most boundary layers are turbulent and that is a good thing, since separation is delayed.
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My feelings have been for years are like robfrey's. It is good on a sit-on bike to have aerodynamic inefficiency with detached flow behind the rider for stability's sake.
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If air almost stays attached or is right on the edge of staying attached at the rear of the bike, it will make for a seriously butt puckered ride as it buffers back and forth between attached and separated, right and left sides of the bike.
It sure does , had a few rides like that
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http://www.landracing.com/forum/index.php/topic,3900.msg51288.html#msg51288
please re-read.
In the areas of transition from laminar to turbulent, gradient optimization is a better choice than trip strips at the higher Rn of LSR. For all the fans of smoke in a low speed wind tunnel, transition gets better with speed not worse.